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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

Calculating the excretion (Ku) and metabolic (Km) rate constants

• Administered dose= 250mg, amount of drug excreted= 125mg, amounts of drug

removed as metabolites 1 and 2 are 75 and 50mg, respectively, and t ½ = 4 h.

 ANSWER

 K = 0.693/4h = 0.173 h
-1

 The % excreted =125mg/250mg x 100 = 50%

 The % removed as metabolite 1 =75mg/250mg x 100 = 30%

 The % removed as metabolite 2 = 50mg/250mg x 100 =20%

-1
 The excretion rate constant (Ku): Ku = 0.173 x 0.5 = 0.0866 h

-1
 The rate constant for metabolite1 (Km1): Km1 = 0.173 x 0.3 = 0.051 h

-1
 The rate constant for metabolite 2 (Km2): Km2 = 0.173 x 0.2 = 0.0345 h

And

K = Ku + Km1 + Km2

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